Phase locked loop (PLL) is a fundamental building block of modern digital and mixed signal integrated circuits (ICs), PLL circuits are used for, among other things, clock generation, multiplication, and signal recovery. Since mixed-signal ICs are typically noisy environments, there has been a trend to reduce analog parts of signal processing and to use digital circuits whenever possible, as digital circuits are more resistant to noise, easy to migrate between technologies, and are easy to test and configure. Typically, only analog-to-digital converters and PLL circuits include analog circuitry. However, a new type of PLL, called All-Digital PLL (ADPLL), has gained popularity. In ADPLL circuits, analog circuitry is minimized, and all signals in an ADPLL main loop are digital. However, a digitally-controlled oscillator (DCO) using a digital word to control the frequency, the counterpart of a voltage-controlled oscillator (VCO), typically comprises an analog circuit.
Accordingly, it is an object of the present invention to provide a digital DCO. Moreover, the present invention discloses a DCO that uses standard digital cells, and is suitable for automatic digital layout tools without any extra specifically-designed circuits. Accordingly, circuits designed according to the present invention, are easy to migrate from one integration technology to another.
Conventional oscillators include analog and digital types which are well-known and are used for many electronic devices. For example, mobile communication devices, such as cellular phones, personal digital assistants (PDAs), and other electronic communication devices, use oscillators to mix signals for transmission and/or reception. Unfortunately, conventional oscillators, such as voltage-controlled oscillators (VCOs), typically rely upon quartz crystals to produce an output signal. Further, these oscillators require the use of analog circuits that requires more space than a digital circuit and are difficult to integrate with a digital circuit (i.e., to form a mixed-use circuit). Therefore, with chip-space at a premium, analog circuits are not desirable. Additionally, quartz crystals consume power and are sensitive to variations in temperature, which can affect the accuracy of an output signal. Moreover mixed-use circuits are difficult to design and manufacture, which can increase cost and design time. Accordingly, it is desirable to provide a digitally-controlled oscillator (DCO) for use with various digital circuits and devices.